Among sensors for measuring and converting physical quantities into electrical signals such as inertial quantity sensors and pressure sensors, in sensors on the detection principle of mechanical displacement, generally, a behavior (vibration mode) different from the steady state is generated by application of vibration at a frequency related to the natural frequency of the mechanical part.
Accordingly, the frequency related to the natural frequency is called a frequency split, and it is important for reliability improvement of the sensors to make robust designs with respect to the vibration application at the frequency.
For example, among angular velocity sensors, one using the so-called Coriolis effect as its principle generally has a configuration of vibrating a mass at a resonance frequency in a drive direction and, when an angular velocity is applied to the mass, detecting a Coriolis force generated in the direction orthogonal to the drive direction from displacement of the mass, and outputting it as an electrical signal. The difference between the resonance frequency in the drive direction and the resonance frequency in the detection direction is the frequency split.
In the above described angular velocity sensor using the principle of the Coriolis effect, it is known that the displacement of the mass in the detection direction becomes larger than the displacement at application of the angular velocity to the band lower than the frequency split, i.e., the band necessary as the sensor by the angular velocity application near the frequency split, and it may be possible that the fault misdiagnosis of the self-diagnosis function of the sensor may be caused by the saturation of signals within a circuit and signal levels outside of the normal range.
FIG. 9 shows open circuit output characteristics with respect to an input angular velocity frequency of an angular velocity sensor without the frequency split (i.e., the resonance frequency in the drive direction and the resonance frequency in the detection direction are equal), and FIG. 10 shows the same with the frequency split at about 500 Hz.
In the case of the angular velocity sensor without the frequency split shown in FIG. 9, its output exhibits a same signal strength characteristic to a certain frequency and the gain becomes lower at the higher frequencies. On the other hand, the angular velocity sensor with the frequency split shown in FIG. 10 exhibits a frequency characteristic having a peak in the frequency split band. This is because the sum of the frequency of the applied angular velocity and the frequency in the drive direction becomes closer to the resonance frequency in the detection direction.
The resonance frequency in the drive direction and the resonance frequency in the detection direction are made equal, and thereby, the frequency split is eliminated, and, as a technology for solving the problem, a technology of changing the resonance frequency in the detection direction by applying a force in a direct current to the mass in the detection direction has been generally known.
Further, in the type of angular velocity sensor, as a technology for preventing output of the signal in the frequency split band with the larger gain, for example, a technology of series-connecting a Butterworth filter and a Chebyshev filter as a filter configuration of reducing only the gain of the frequency split while maintaining the gain in the desired band of the sensor has been known (see Patent Document 1).